The present invention relates generally to the field of pneumatic control devices and systems, and more particularly to pneumatic thermostats used to vary a pressure or flow control signal in a pneumatic system based upon a sensed temperature.
Many systems are pneumatically controlled based on a gas (generally air) flow or pressure. Pneumatic control systems tend to be simple and robust, operating over a wide range of temperature and other environmental conditions, and have long working lives. Pneumatic control systems reduce the likelihood of fires, because air is non-flammable. Pneumatic control systems pose no danger of electric shock, present no possible failure mode due to electrical short circuiting, and are relatively unaffected by electro-static discharge. Pneumatic control systems are mechanical devices that may avoid the need for a power source (in short or long term), thereby reducing wiring requirements, and may be unaffected by electricity fluctuations or power outage. Pneumatic control systems are environmentally friendly, as leakages in the system do not negatively affect the outside environment or cause pollution.
The pneumatic control signal can be determined on any number of sensed input parameters, with a commonly sensed input being temperature, in which case the pneumatic control device is commonly called a pneumatic thermostat. Pneumatic thermostats convert a substantially constant pressure air supply into a pressurized air signal indicative of sensed ambient temperature. Pneumatic thermostats are used in many pneumatic systems, with a common use being to control variable air volume (VAV) units (such as disclosed in U.S. Pat. No. 4,957,238, incorporated by reference), ventilators, radiators, coil units, fans, dampers, valves and the like in heating, ventilation, air conditioning (HVAC) systems. Pneumatic thermostats are still reliable workhorses that control temperature in many older buildings across the country.
One example of a pneumatic thermostat is shown in U.S. Pat. Pub. No. 2011/0166712, incorporated by reference, which combines a valve unit (typically a diaphragm type valve), a “flapper” controlled nozzle, and a bimetallic strip. Supply air is passed through the valve unit, which controls the pressure at an outlet, after allowing a portion of the supply air to exit into the atmosphere through the flapper-controlled nozzle. The bimetallic strip is linked to the flapper, and the position of the flapper over the control nozzle (and hence the outlet pressure) moves responsive to changes in temperature witnessed by the bimetallic strip. While pneumatic thermostats such as described in U.S. Pat. Pub. No. 2011/0166712 provide certain advantages, they tend to have reduced accuracy particularly associated with the bimetallic strip.
Largely separate from the pneumatic thermostat industry, the field of piezo electrics is becoming more well-known. Piezo electrics are advantageous in that the power consumption is low. One of the rare instances in which piezo electrics are applied to a pneumatic thermostat is shown in U.S. Pat. No. 8,527,099, which teaches an embodiment using a piezo electric device as a prime mover controlled by a bimetallic strip based pneumatic thermostat.